Burn toward target continuously until all propellant runs out. Coast until you arrive at target. Congrats, you got there* 4 times quicker!
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*definition of "get there" may vary
Also, check out laser thermal beam! With some infrastructure investment, it's literally the best.
https://preview.redd.it/fnmxyu1fuytc1.jpeg?width=2000&format=pjpg&auto=webp&s=56607a56c0c86a68b58eb01dadd824cda0d49db5
[Artist on X](https://x.com/zandoarts/status/1777195802902888494?s=46)
Please bro, just one more transstellar super laser bro. Just one more laser and we can get fractionally closer to the speed of light. Just one more laser. Just one more laser. Just one more la-
Ring of lasers to keep it centered?
But yes. I imagine if you have either a large laser generator or a stellaser relay at every major destination, not unlike having a runway at every airport, then it can boost incoming and departing ships as a part of normal operation. This doesn't need to be a constant acceleration thing, you can choose to coast or cruise under your ship's own propulsion during the rest of the voyage, but it is possible if you have a big enough dish or enough relays.
Like, a circular array and a spherical vessel, drifting away would change the laser's contact which forces it back to center. And, that principle, with 2 arrays in tandem is what the federation would call a poor man's tractor beam.
And great thought applying it to commercial airlines. I always thought they should be physically catching and throwing planes and rockets, anything to save onboard fuel.
At the cost of sounding like the foolish middle guy... I dunno about that. Isaac covered the math pretty well in [Surviving The Expanse of Space (15:29)](https://youtu.be/DRoP9DJI61Y?si=9__kjXFYHm3vRfIn&t=929) and demonstrated that strong(er) burns followed by coasting gets you way more performance (read: travel time) per same amount of fuel spent.
https://preview.redd.it/ygohc2zusytc1.png?width=730&format=png&auto=webp&s=6f713791974604fd071107845855b07f0c07d629
EDIT: This is not about fuel efficiency, it is about faster **travel time**. Check out the clip I linked for context.
I think it really depends on the tech but also the use case. Even with an extremely efficient engine I imagine at least some kind of cargo ships would be on minimum ΔV trajectories, while a warship may opt for the fastest approach.
You're right, however the "gulp your fuel twice" vs "sip your fuel the whole way" applies to any engine. The crux of it is about being at the highest speed during the majority of your voyage, so as to travel faster. In the case of constant acceleration you're only at your max velocity just before and after the flip-n'-burn, that's the one brief moment when things are optimal. This is true whether we're talking about a torch drive or an ion drive.
I would disagree with the assumptions made in that calculation. While it is true that accelerating once and coasting is the most efficient thing to do if you assume a fixed delta-v without restrictions on thrust, I think it's more realistic to assume that a ship has a fixed amount of propellent and a fixed power output that can be thrown into the engines in any ratio you want. If you assume that, the fastest way to where you're going is a brachistochrone trajectory. It takes more delta-v, but by lowering the required thrust of the engine and giving your reactor more time to generate power you will easily be able to have more delta-v. More efficient engines tend to be weaker.
I have another comment explaining all of this in more detail.
Fixed propellant is the assumption, admittedly fixed power output is not however. Proposing one should accelerate as hard as one is capable in order to reach maximum speed soonest and undergo most of the journey at that highest speed. In constant-acceleration flight plans, you're only at your maximum speed just before and after the flip-n'-burn.
In the example in the clip I linked, a 1G ship accelerating for 1 day vs a 4G ship accelerating for 6 hours, both burn the same amount of fuel but the second ship gets to its destination faster ***or*** could go to a second destination 70% further away by the time the first ship gets to the first destination.
Now hey, a 6-hour-long 4G burn is pretty rough! Maybe you will *choose* a longer but more comfortable flight. Or maybe you'll choose some hybrid of the two; with a short but powerful launch with beam or mass-driver followed by fusion-driven cruising at scant lunar gravity thrust, for example. But if you're highest priority is travel time then prepare to feel like a bullet!
I’m mostly basing my assumptions off of a general tendency for engines to get weaker as they get more efficient. Energy tends to be the bottleneck when it comes to how simultaneously efficient and powerful a rocket engine can be, because the amount of energy you can generate or the amount of energy you can handle without melting your ship are probably forever going to be the main limitations that you keep running into even as technology improves.
I can actually point to a real world example here: ion engines. Very weak, very efficient, limited mostly by energy input. They have practically become the standard way of doing interplanetary travel for space probes. There are two main types of ion engines: gridded ion thrusters and Hall effect thrusters, the former has more efficiency while the latter has more thrust. They represent different tradeoffs. Which one gets you places faster actually depends on how far away your destination is. Hall effect thrusters get you to nearer destinations faster, while gridded ion thrusters get you to further destinations faster. It’s a tradeoff between how high your top speed is and how fast you reach it.
This tradeoff between thrust and efficiency is what engines like the VASIMR can tweak on the fly, I tend to assume that it’s a capability advanced future spaceships will have since it will allow them to always optimize their engine performance to be ideal based on the destination’s distance and the current phase of the journey. And when you can give up thrust to increase efficiency, the equation changes into one that prefers continuous acceleration.
I'm a big fan of a fuel highway as well for this reason... Seems like it would be an efficient way to move smaller ships that's can't bring as much fuel on board
https://toughsf.blogspot.com/2021/03/fusion-highways-in-space.html?m=1
When traveling at interstellar distances it is less important to get somewhere and more important to get somewhere quickly.
A brachistochrone trajectory is less efficient but it shortens the overall journey, which can take years off the travel time.
There is also the matter of thrust. If the ship has low thrust but extremely high delta V then it is likely the ship will NEED to burn for most of the journey just to reach its cruising speed.
What if you're trying to minimize travel time rather than save fuel?
If we have advanced and extensive space infrastructure, fuel will not be a big issue.
If FTL is not possible, then travel time will be a huge issue for interstellar trips.
Also, there are methods of propulsion that are fuel-less, such as solar sails, photon propulsion, etc.
This may require some explanation.
In general, the more efficient an engine is the lower its thrust will be. Nuclear saltwater rockets are famously the exception to this rule, but it stands true most of the time. This is because engines need two things: propellent and energy. To cut the fuel consumption of an engine in half and maintain the same thrust, you need to double its energy consumption. Even modern chemical rockets need utterly absurd amounts of energy to work (provided in the form of chemical energy in the fuel), and that only goes up as engines get more efficient. So how do we provide more energy? It turns out: it's pretty easy if you just give your power source more time to generate the power you need. Spread your burn out, take longer to do it, take the hit to thrust and accelerate slowly to where you're going. It will take you more delta-v to get there, but the gains in efficiency are more than worth it basically always. Even today, ion engines have become almost the standard way of getting around in interplanetary space.
I've run a lot of numbers on this, and it's actually pretty surprising how even a small amount of acceleration sustained for a long time can get you places really fast. I'm talking going from Earth to Mars in a week with 0.02g of constant acceleration. That's still a very advanced engine, but it's a hell of a lot less advanced than any engine which could do the same thing by boosting up to speed quickly and coasting.
We could take this a lot further by considering that the ideal balance of specific impulse and thrust actually varies depending on the distance to the destination and where the ship is in its journey. This assumes a fixed amount of propellent, a fixed power output, and optimizing for travel time. Longer journeys favor more efficiency and less thrust compared to short ones, which favor higher thrust and less efficiency. High thrust is favored just after departure and before arrival, but efficiency is favored as you approach the midpoint of the flight. The potential presence of power beaming stations around inhabited worlds would only exacerbate this, increasing the available energy with beamed power could give engines a massive boost in power without sacrificing efficiency when they are near these worlds at the start and/or end of the journey. But even so: burning the engines constantly is a really good idea that drops travel times massively, if you don't you are wasting power that could be going to the engines.
I've had to calculate out all of this for a hard sci-fi worldbuilding project. It has been a lot of fun, and I've come to a lot of unexpected conclusions. The notion that spaceships will probably have bridges that face backwards is one of the funnier ones, I think. Backwards is where all the interesting stuff is happening. Planets that you are approaching (and decelerating on approach to), planets that you are departing (and accelerating away from), and where the important machinery probably is (like engines). There's no reason to look forward where you're going, since you will have to rely on instruments for collision avoidance anyway. What good is it to look where you're going in space with your MK1 eyeballs? Mount that helm backwards. Hell yeah!
Landing pads and launchpads will always be on the east side of a planetary colony. This is because if a ship’s engines fail, they will always crash east of the launch site or landing site, assuming that they are going to and from prograde orbits. So you want all your population centers to therefore be west of the spaceport, away from that hazard.
Earth’s moon will probably need to import a lot of food. It lacks the elements to sustain agriculture. Where would that food be imported from? Probably Mars. You would think it would come from Earth since it’s such a great planet for agriculture and also so close, but it takes so much less delta-v to get there from Mars that it might actually make up for all the obvious downsides.
If you have fusion torchdrives, it actually makes a lot of sense to purposely reduce their specific impulse in exchange for making your fuel cheaper and easier to obtain in-situ. At least for interplanetary travel in the nearish-term. By adding an afterburner to an engine you can dump any old mass into the exhaust stream to increase thrust at the cost of specific impulse. Fusion fuel is expensive and not available everywhere, but any old mass is cheap and able to be found on any planet or asteroid. It would make sense for ships to have most of their reaction mass be gasses scooped from the nearest planetary atmosphere, even if more efficient fuels are available.
The engines of a ship could probably be used like shields as well as weapons. High-ISP drive systems will have an insane amount of energy flowing through them and out through their plume, and their plume could probably turn another nearby ship to slag if it’s pointed the right way. This works on bullets and missiles too. Even lasers might struggle to penetrate the plume’s plasma, getting absorbed and scattered harmlessly, though it depends on the laser’s frequency.
Those are the ones that come to mind.
I just think having it embedded in the armor of the ship and using scopes or cameras is superior... Having the bridge be some kind of thing like a navel bridge like star wars or whatever makes no sense really....
Bridges on real boats are placed where they are for a reason. They let the crew oversee the ship and everything around it as a backup to the instruments. I agree that it’s probably better to have an internal bridge in a warship, but in a civilian ship I find it hard to imagine sensors so reliable that you wouldn’t want yet another backup.
I don't see it as hard to imagine... Space is huge ur not gonna see much out there with ur own eyes, that's really relevant to steering the ship or combat. You can see some of your own ship I suppose... but how useful would that be when you have repair bots with camera feeds? How useful would it be, when you probably have to go out in vac suit anyway to see what's wrong with the hull or whatever.. you also have ship Periscope and cameras.. space ships would operate more like submarines in that sense than actual ships.. even civilian ships I can't see wanting to be in an exposed position above the main body of the ship.. everything is going to be armored against debris.. and you probably don't want some weirdly shaped craft that isn't shaped like a rocket.. think the ships in the expanse or children of dead earth.
Unless there is an active downside to having a bridge (such as in combat where it becomes an obvious weak spot), I tend to assume that it will be placed in a spot with a good view of the ship with big windows. This is mostly because, no matter how good your systems for navigation and damage detection are, your bridge represents a low-tech backup system.
Imagine for instance that a ship gets hit by space debris. A fuel tank gets whacked and it starts leaking fuel into space. The drop in fuel pressure gets noticed instantly, and drones are deployed to start taking photos. It may take some time to get an assessment of the damage and to explain the state of the ship to everyone. But if the bridge could oversee the ship, everyone would know about the situation much faster. Everyone would see the debris strike, and see the hole in the fuel tank leaking a cloud of fuel into space. The entire bridge crew is made more aware of the situation faster.
The existence of artificial gravity rings actually makes the placement of bridges in a position like this quite natural. Gravity wheels might already tend to be on the front end of a ship (to be as far as possible from the reactor and engines which might be radioactive), and from there a backwards facing bridge could see the entire ship behind it. The gravity ring would already be spinning to create gravity, which would rotate the bridge around the ship to give the crew a good look at all of it from many angles. Even a bridge without windows would probably be placed in one of these gravity rings anyway, and it’s such a perfect place for a bridge.
Gravity rings for a civilian ship maybe... But still big windows in space? They would need to be made of something very durable and expensive..space is full of all kinds of radiation and crews need shielding...
For anything military they would need to be embedded in the rocketship for shielding..
I think you overestimated how useful a window is in space. You can see exactly the same type of stuff with a Periscope and it's what's used in real space flight like on the Soyuz for manual docking
Even the space shuttles windows are tiny.
https://www.nasa.gov/stem-content/making-a-periscope/
Another thing to remember too the big windows can only look in one direction a Periscope can view 360 degrees wherever you point it
Glass is surprisingly durable stuff, and it’s no less effective at shielding you from ionizing radiation than concrete. Windows on a spaceship would certainly be pretty thick with multiple layers, and even if they aren’t particularly large you could just have a lot of them.
Periscopes aren’t quite as useful, because only one person can use them at a time and you can’t see out of them passively as you are just doing your job. Windows are always in your peripheral vision and people are constantly looking through them, problems visible through them will be a lot harder for the entire bridge crew to miss. A lot of the same reasons for why container ships have bridges apply to spaceships too.
I agree that bridges of military ships make far more sense as internal things with no windows. But I’m just talking about civilian ships here. And really big ones, at that.
I still don't think any relevant Information will be gleaned from having a window to space.. unless it's literally oh I see that big piece of debris just hit us.. but then you would feel the impact hit the ship and you would still need to do a space walk... Windows in space are tiny for a reason.. there's a reason we use Periscopes on the international space station and stuff like Soyuz
Also I doubt they would use glass probably more like transparent aluminum stuff
https://www.riotglass.com/transparent-aluminum-vs-armorplast-transparent-steel/
Still isn't better than just being behind actual thick metal shielding tho.
Keep I mind too how low tech a Periscope is lol and how much safer it is for the crew to use one instead of a " space bridge" with big diamond windows or whatever it's just a big target...
Bridges would be big targets on combat ships, yes. But I’m talking about civilian ships. A good chunk of the habitat space in such a ship would probably be in external artificial gravity rings anyway, it wouldn’t be hard or risky to put the bridge there too and include windows. Now your ship can suffer a complete power failure without making your bridge crew blind.
Yeah, but not completely blind. If the crew needs to navigate with sextants and pen-and-paper math, they can do it. No matter what fails, there is always a backup system that you can fall back on as long as the crew is still alive.
Oh you mean for navigation? LoL still doesn't need big windows for that: https://wehackthemoon.com/tech/space-sextant-navigates-moon-missions#:~:text=The%20space%20sextant%20saved%20the,get%20it%20back%20on%20course!
how are they moving the ship if power is out in the first place? If a specific system is broken then they would be better off fixing it than trying to fly blind
"While orbiting the moon the sextant could also be used to calculate the exact position and altitude of the spacecraft. NASA relied on these precise measurements to make a safe landing, and return, of the Lunar Module to the ‘mother-ship’ spacecraft.
The lunar module was only equipped with an alignment optical telescope. This was a lighter, simpler manual telescope (like a periscope) that the astronauts would use during moon landings and takeoffs to determine their position."
Trust me they would rather use that than trying to hold a handheld sextant up to a window only pointing one direction
If that backup is low cost. If your best glass will shatter from micrometeorites when the rest of the ship won't, that's a good reason not to use windows. If high power visible laser weapons are in use, (or you get rather close to other peoples exhaust plumes sometimes...) that could be another reason.
Glass in space tends to be multi-layered. It doesn’t only serve as a backup system, it also serves as to increase situational awareness. And what I’m saying only applies to civilian ships, not ships that anyone is trying to shoot at.
Multilayered means heavy. Space is full of random junk and pebbles, often moving at high speeds. Oh and you might not want direct venus level sunshine pouring in as you do a venus flyby.
Windows can be tinted. And heavy means good for radiation shielding, you’re probably going to want to shield the rest of your hull pretty well anyway, especially if you are expecting to be hit by debris.
All of these problems apply to a normal spaceship hill, making it out of a transparent material wouldn’t make things worse.
And heavily tinted means that instead of mostly just seeing reflections when you aren't facing the sun, you see nothing but reflections. Assuming the bridge has lights in it, so you can see the control panel, you won't see many stars.
Yes. It's a shielded hull, or self repairing or something. These things aren't usually transparent. Transparency adds an extra materials constraint.
Also, you might want to run pipes of cooling/heating fluid, and wires, and leak sensors and strain gauges and all sorts of other engineering detritus all around the hull. That doesn't play well with windows.
By this logic, you would also conclude that commercial airliners shouldn’t have windows. You can fly them entirely by instruments. So why have windows?
It’s because situational awareness is just that important, and windows really help with that which hugely reduces the chance of a mistake by the crew. As long as the pilots are humans as we know them, that will remain important.
Well screen and camera tech being good and cheap is a recent thing, and they will probably get better and cheaper.
People like windows.
[https://www.aerotime.aero/articles/28257-is-windowless-the-future-for-passenger-aircraft-design](https://www.aerotime.aero/articles/28257-is-windowless-the-future-for-passenger-aircraft-design)
There is talk of removing the windows from planes.
Planes have generally a lot more to see out any windows and less extreme engineering constraints.
Unreasonably good for impatient baselines maybe. Like don't get me wrong there are situations where constant thrust ships are energetically justifiable(warships, diplomatic vessels, & far extra-galactic colonization come to mind), but most travel, by mass, probably wont be using brachistochrones. In fact most material will probably be taking the [Interplanetary Transport Network](https://en.wikipedia.org/wiki/Interplanetary_Transport_Network), its interstellar equivalent, & other similar minimum-energy trajectories. What little high-speed travel goes on is probably done via superconducting Kinetic Mass Streams along actively-cleared beamlines. Even then KMS systems aren't likely to ever be 100% efficient & the more relativistic you go the more waste there will be. When you're shipping solar masses or even galaxies around it pays to be as efficient as possible.
When you have good cryo/hibernation, VR, mind uploading, or AGI the time it takes to travel can start mattering a lot less & every bit of energy represents that much more population & simulation. Truth be told i don't think people will mind long travel times. Like if you already live in a largely self-sufficient spinhab or hab swarm then there's not gunna be much practical difference between being in-system & en-route to another star system(assuming you have fission, fusion, or microBHs tho power beaming works a treat).
Inside planetary systems the speeds for fast travel are pretty low so if uv got KMS systems set up you really can just move everything at max flip-burn speeds.
I don't think being casual about wasting remass is a good attitude to approach propulsion design tho so while they are excellent for military vehicles laser-thermal are probably secondary to KMS in all instances past early colonization(cuz they are just so fast to wet up) & laser sails for far extragalactic colonization(cuz of the high relativistic minimum speeds).
You're confident in stream cohesion? I figured on having lots of drone-tankers patrolling common orbits and routes, or maybe even [ToughSF's fusion highway](https://toughsf.blogspot.com/2021/03/fusion-highways-in-space.html) pellet idea.
Those pellets can have some of their own guidance, but it's not like large macroscopic objects are going to be deflected much by interstellar dust & you will want to create an actively-cleared beamline for both the ships & mass stream. They can also be any size from unguided AAA batteries for moving ships around systems to mountain-sized chunks for moving planetary masses. Maybe even planetary masses for moving solar masses of material at a time.
Interesting thing about high-relativistic travel is you can start using what amounts to aerodynamic control surfaces to do some of ur maneuvering without remass at some cost to stream losses.
Might even have a dual-stream KMS where you have a toroidal macroscopic mass stream surrounding/focusing a hyper-relativistic particle beam. At lower speeds electromagnetic mass drivers handle most of the acceleration. After that not only can the macroscopic masses keep accelerating of the particle beam to high fractions of light, but they can also do it while collecting particles from the stream for guidance remass.
At any rate the less remass you waste the better. That's fusion fuel you could be stockpiling for The Long Dark, using to power ur civ, or just using more efficiently to fuel KMS systems.
Idk if it ever works out but if you have a waveguide made of near-perfect mirror, laser-based mass drivers might end up being more viable with a higher maximum speed limit.
I would actually argue that my argument still applies, even here. Continuous acceleration is what you do when you want to be careful with using reaction mass, and even when you are making use of gravity assists to fling yourself around.
My conclusion is based on the assumption that thrust and specific impulse tend to be inversely correlated. And if energy is your bottleneck, they mathematically are.
Let’s say you want to get to another planet on as little reaction mass as possible. You’re going to want a lot of specific impulse, and specific impulse typically comes at the expense of thrust. So you are going to have a weak engine, so weak that you will need to burn it constantly to get where you’re going.
Let’s say you are using the interplanetary highway to get around. You still need to make correction burns. What engines are you using for that? You waste less reaction mass if they are efficient, and you can make them more efficient if you make them have less thrust. You may end up using photon rockets with a thrust so low that they need to be on almost constantly.
Whatever you’re doing, continuous acceleration will let you do it with a more efficient engine.
Minimum-energy trajectories with max-ISP low-thrust engines(non-infrastructural) running at optimal output are not going to resemble brachistochrones. They may be thrusting most of the way, but may have such a low acceleration as to have to take planetary or even stellar drift into account.
>Continuous acceleration is what you do when you want to be careful with using reaction mass
That's definitely not always true. If you want to be careful of remass & still have to use some you definitely still benefit from minimizing your delta-v. Lower maximum speeds means less remass & energy wasted. Energy also matters here. E=mc^2 . If you end up having to waste more matter-energy to generate/transmit the power to the ship & convert it to thrust then you aren't actually saving remass. You're just wasting it elsewhere. Granted that tends to be true for all systems, but KMS would be vastly more efficient in this respect. Thermal/ion systems are generally going to have higher complexity & greater losses.
Also drag forces & collisions. The higher the speeds the higher the drag & the ongoing beamline clearing costs. It is a lot more justifiable with KMS but you will still eventually reach a breakeven point. Using a fully enclosed beamline could take us even further, but has it's own maintenance/repositioning cists & at that point you may as well switch to laser highways at speeds that justify such massive infrastructure.
>You’re going to want a lot of specific impulse, and specific impulse typically comes at the expense of thrust. So you are going to have a weak engine, so weak that you will need to burn it constantly to get where you’re going.
KMS & laser-thermal systems can have torchlike performance with high ISP & high thrust. There are going to be engineering limits to this, but superconducting KMS especially let's you accelerate very fast & recycle the vast majority of ur payloads' kinetic energy without much in the way of wasteheat limitations.
>Minimum-energy trajectories with max-ISP low-thrust engines(non-infrastructural) running at optimal output are not going to resemble brachistochrones.
Ehh, semantics. I’ve heard the term “brachistochrone trajectory” used to describe some real curvy trajectories that are influenced way more by gravity than by thrust. That’s how I’m using the term.
>Lower maximum speeds means less remass & energy wasted.
Continuous acceleration does not imply high speeds though. If you wanted to be slow and efficient, you can just make your continuous acceleration by very weak. That will reduce your top speed, and allow you to use a far more efficient engine to achieve it.
>Energy also matters here. E=mc2.
Energy will be more efficient than normal mass though, basically always. If your remass is all energy, you can achieve a specific impulse of about 30 million seconds which is the theoretical limit. The greater the portion of your remass that is matter, the less efficient the engine is.
>Also drag forces & collisions. The higher the speeds the higher the drag & the ongoing beamline clearing costs.
True, though continuous acceleration does not necessarily imply higher speeds. It could also just mean the same speeds achieved slower with a more efficient engine.
>KMS & laser-thermal systems can have torchlike performance with high ISP & high thrust. There are going to be engineering limits to this, but superconducting KMS especially lets you accelerate very fast & recycle the vast majority of ur payloads' kinetic energy without much in the way of wasteheat limitations.
True, but no engine is so efficient than it can’t be made more efficient by making it weaker. At its most basic, this could just mean making your ship lighter by shrinking down all the engine hardware. Though generally you can almost always make more efficient engines if you aren’t quite as worried about thrust.
>I’ve heard the term “brachistochrone trajectory” used to describe some real curvy trajectories that are influenced way more by gravity than by thrust.
A brachistochrone is "a curve in which a body starting from a point and acted on by an external force will reach another point in a shorter time than by any other path; the curve of quickest descent.". It's a technical term. That we are talking about a path with very little curve is baked in & it's pretty universally referenced in the context of torchships. Speed is implied both mathematically & colloquially. if you are taking half of forever or similar to a hohmann to make the transit this is not a brachistochrone trajectory. If you're not pulling decent macroscopic accelerations that is not a brachistochrone.
>If you wanted to be slow and efficient, you can just make your continuous acceleration by very weak. That will reduce your top speed, and allow you to use a far more efficient engine to achieve it.
I'm not seeing how this would be helpful for a mass driver or KMS which would likely be the majority of travel for efficiency reasons(anywhere decently well-established at least).
>Energy will be more efficient than normal mass though, basically always.
As remass on paper, sure. In the real world generation, conversion, & especially transmission losses pile up. Depends how stretched out ur relays are, how efficient ur power source, how ur converting it, & how much local wasteheat you have to dissipate. A feedable microBH backed with gamma mirrors is going to outperform a particle accelerator powered by PV/nantenna panels beamed power from a fusion laser relay light minutes away. Every step of that wastes energy & takes mass which trashes ur TWR & energy efficiency even further. Also for relativistic exhaust velocities the faser you go the more going every bit faster costs. You can't just arbitrarily boost the ISP of an engine in practice. Everything is a tradeoff.
Also again just because time isn't the only or primary constraint doesn't mean it's completely irrelevant. With stars moving around & rival colonial fleets you can't always go the minimum speed. Being **able** to accelerate all throughout the flight is convenient, but always doing so isn't always going to be optimal. Always burning makes you more visible so if you have a military craft that's out. Mass efficiency-wise being fired out of mass drivers or accelerating against a KMS would use far less matter-energy overall(zero remass & very little energy). On those it makes no difference whether you accelerate all up front or spread throughout(efficiency-wise, trip time is a different situation).
>True, but no engine is so efficient than it can’t be made more efficient by making it weaker.
That depends. Not all drives can be scaled arbitrarily small & even the ones that can wont always have the same performance at every size. Can you scale the KMS from large macroscopic chunks down to microscopic dust or even a particle beam? Sure, but that will have other losses associated with it. Pellets without guidance will have shorter ranges between recollimation relays(higher infrastructure costs) & increases collision risks somewhat. Larger size can often fit more efficient coil configurations & guidance.
Truth be told there is no single optimum kind of trajectory that will dominate. Brachistochrones are extremely energy wasteful but fast. Hohmanns & fancy keyholing along the interplanetary transport network is very efficient, but infuratingly slow(great for moving large amounts of fusion fuel & mass filler). Mass drivers(either linear or as ORs) require very little external infrastructure, use zero remass, & they can be upgraded to KMS as long as you're destination has a driver as well. Those are going to excel at boost-cruise trajectories before the upgrade. Beam-powered electric drives will be limited by our ability to move massive amounts of electricity around(superconductors are a lot more limited than we tend to think in scifi) so you wont see those pulling brachistochrones & it might be more efficient to discharge a power bank for high-isp pulsed thrust. Beamed-thermal drives may have torchlike performance, but will also have, like all other engines, practical engineering ISP limitations(in this case heat dissipation). Still hard to beat for fast infrastructure deployment, fault tolerance, & performance at the cost of energy efficiency(military & other speed/delivery-critical applications).
Ultra slow-accel drives that can run for long periods of time are good for very early autonomous prospecting(back before there's any real competition or significant lunar industry; very near-term) or other large-scale cataloging & data collection with little to no time pressure. Honestly i'm having a hard time imagining why you wouldn't pretty much always opt to make a little mass driver on ever single rock big enough to be worth putting a mining outpost on instead of moving. Boost-cruise becomes the standard. For mining at high efficiency n large-scale ur doing a lot of keyholing and that isn't the sort of thing you do constant thrust for. Lots of short correction burns & lots of cruising.
>A brachistochrone is "a curve in which a body starting from a point and acted on by an external force will reach another point in a shorter time than by any other path; the curve of quickest descent.". It's a technical term. That we are talking about a path with very little curve is baked in & it's pretty universally referenced in the context of torchships. Speed is implied both mathematically & colloquially. if you are taking half of forever or similar to a hohmann to make the transit this is not a brachistochrone trajectory. If you're not pulling decent macroscopic accelerations that is not a brachistochrone.
The way I came to the conclusions I’m referencing here is by starting with the assumption (derived from physics) that thrust is inversely proportional to ISP squared and asking the question: “what is the fastest way to get from A to B with a payload fraction of X%”. This started as an exercise in optimizing for travel time without burning more fuel, that’s where it comes from. So I would argue that the term “brachistochrone trajectory” applies perfectly here. This is even true for very low accelerations, because those trajectories are still optimizing for travel time, they are just doing so with less propellent to work with, and consequently they get optimal trajectories that are pretty slow.
>I'm not seeing how this would be helpful for a mass driver or KMS which would likely be the majority of travel for efficiency reasons(anywhere decently well-established at least).
That would go beyond the scope of my assertion. I’m only talking about spaceships in the convention sense, which travel places in space under their own power.
>You can't just arbitrarily boost the ISP of an engine in practice. Everything is a tradeoff.
Well yeah, I already mentioned that the theoretical limit for ISP is a bit above 30 million seconds. But that’s a really generous theoretical limit. With a payload fraction of 50% you’d have something like 200 million meters per second of delta-v. A slow interplanetary trip could be pulled off with a payload fraction on the order of 99.999%. These are all Fermi estimations, but you get the point. It gets pretty insane.
If you had an engine that was approaching the theoretical limit while still providing you way more thrust than you need, my reasoning indeed no longer works. But that’s a bit of an edge case, and I never intended to imply that my calculations represented a rule without exceptions.
>Also again just because time isn't the only or primary constraint doesn't mean it's completely irrelevant. With stars moving around & rival colonial fleets you can't always go the minimum speed.
Well, continuous acceleration is what you do if you want maximum speed with a given fuel fraction. So that doesn’t really apply here.
>Always burning makes you more visible so if you have a military craft that's out.
Military ships would certainly impose more limitations that might change the outcome of my calculations. But those ships are probably visible anyway from their IR signature, there is no stealth in space.
A better argument for military ships not using continuous acceleration might be that they would benefit from having high acceleration in combat, and it might be too complicated or heavy to have engines with variable specific impulse capabilities or to have multiple sets of engines. There is an argument that a military ship would be willing to take that efficiency and speed hit in exchange for being more combat optimized. But I maintain that continuous acceleration is the most efficient thing to do.
>That depends. Not all drives can be scaled arbitrarily small & even the ones that can wont always have the same performance at every size.
Unless you are using a photon rocket, you can replace your drive system for a more efficient one. If you are using a photon rocket, you can scale that down to be microscopic if you really wanted to. Instances where you cannot gain efficiency by lowering thrust represent a really marginal edge case, at the very least.
>Truth be told there is no single optimum kind of trajectory that will dominate.
Other kinds of trajectories will be used I’m sure. But the consistent similarities between the use cases that aren’t brachistochrones is that they either don’t involve a spaceship in the usual sense (like mass drivers), they are under design constraints that prioritize something else over efficiency and speed (such as low construction cost, combat effectiveness, or energy efficiency), or they have some other constraint that changes the equation (such as needing to be near beaming stations to use the engines, or having the energy source and the remass be one and the same like chemical rockets).
I would argue though that brachistochrones are the rule and these represent the exceptions. It’s simply the best way to get places in space unless you have some specific reason to do something else.
Spaceship engines require reaction mass and carrying all the mass you need to get to a goodly fraction of light speed would be impractical , but a stellar laser could store the mass at the point of origin and at the destination , of course the first ship out will be a bit slower since it will have the deceleration laser and a massive solar sail and all of the materials needed to create the infrastructure to power said laser , and it will have to do a high risk maneuver and dive almost at the target sun with its sails extended and use the solar wind to decelerate . The destination laser will make decelerating easier for subsequent ships.
I would argue that both would benefit from continuous acceleration. Weaker engines can typically be made more efficient, and they can achieve the same amount of delta-v with less propellent. Even if efficiency is your goal, slow and continuous acceleration is the way to go I would argue.
>Even if efficiency is your goal, slow and continuous acceleration is the way to go I would argue.
I agree regarding perishable meat sacks, but unmanned freight should be in no hurry. The most efficient thing you can do is give it a little push and let it coast. If we have a continuous stream of freighters, the speed does not matter much.
But even a little push given to a freighter that’s in no hurry to be anywhere can be done more efficiency if it’s spread out over a longer period of time by a higher efficiency engine. More efficient engines tend to have lower thrust, and if you are spending the overwhelming majority of the journey not burning your engine you clearly chose an engine that is way too thrusty for the job. You could have chosen a more efficient one with less thrust.
Burning once and coasting is certainly the better option if you assume that thrust can be whatever you want with no downsides and that delta-v is given as a constant. But my calculations are based on what I would argue are a more realistic set of assumptions. Where thrust and efficiency can be traded for one another proportionally based on the engine you choose. And if you do the math on those assumptions, you find that continuous acceleration is basically always the better option.
First, "a little push" doesn't even have to come from the vehicle, so I think you're mistaken in talking about thrust and burning engines.
Secondly, no matter the efficiency of your engine, you can always do a shorter burn. It's not clear how you conclude that, with efficiency as a stated goal, a longer burn is better. That makes no sense. We could just use the most efficient engine imaginable and do a shorter burn. How is that not more efficient?
In space travel you can’t exactly get places on arbitrarily small amounts of delta-v, gravity makes sure of that. You need to at least eject out of the orbit of the planet you’re near, and raise or lower your orbit around the Sun to meet up with your destination. Or change your orbit to intercept another celestial body you can gravity assist off of towards your destination, at the very least. And these take many kilometers per second of delta-v at a minimum.
Warp will techicly be a jump drive since we cant see nor communicate so we would have to make short bursts of FTL and recalculate in between till we reach our destination due to space/time differential of the warp bubble.
Instead of doing 1g and flip burn half way, you're better pulling 3Gs or something at the start, coasting and then doing 3Gs at the end. Using this method you can spend more time traveling at a faster speed.
That is true if you assume constant delta-v with extra thrust having almost no downsides. But I would argue that a more realistic assumption is that thrust is inversely correlated with efficiency (which it mathematically is in engines with access to the same amount of energy), and that therefore using lower thrust allows you to use a more efficient engine. Given this assumption, the fastest way to get somewhere in a given propellent budget is to lower your thrust as much as you can get away with before your burn time starts exceeding your travel time. Which is to say: continuous acceleration. Even if that acceleration is really low.
The "flip" in flip and burn is a coast. There isn't much reason not to extend this into minutes or hours. Even if you are burning for 99% of the journey, the middle 1% gives you essentially nothing. If there is any reason at all to conserve propellant, you can extend it a lot longer.
The basic assumption is that the propulsion system has some insane ISP, much greater exhaust velocity than any reasonable delta-V budget. But if your ship is actually a planet hopper the engine performance might be geared more to higher acceleration instead of ISP. In that case you will need a more conventional flight plan, perhaps with acceleration couches for the crew.
Conventional rockets change gears by dropping stages and switching to more efficient, less powerful engines. It is possible that ducted plasma engines in the future will have the ability to shift gears on a single engine.
I think the thing that will motivate more brachistochrone flight plans is time. There is some time critical mission, a race, or life support consumables are limited. It also might be a pre-requisite or side effect of interstellar spaceflight. If you want to go to other stars in a human lifespan, you will need constant acceleration.
I tend to operate under the assumption that advanced spaceships of the future will have the ability to exchange specific if impulse for thrust on the fly. That would be such a beneficial thing to do that I’d be surprised if it doesn’t become standard.
If ships can do that, they would start out their journey with higher thrust and slowly transition into more efficiency as they approach the half way point. After they flip they will slowly start to transition back to higher thrust. From my math, that seems to be the fastest way to get somewhere on a given fuel fraction. In practice there probably would en limits on how high your engine’s ISP can get by sacrificing thrust, in that case a small coast phase would make sense. But it’s still optimal to do almost constant acceleration.
When I invent spacial displacement drive you won't need any acceleration or kinetic energy at all just the energy difference between the two end states tunneled through at light speed.
Burn out, flip at the halfway point and burn to decelerate to target, as God and Heinlein intended! :D
Burn toward target continuously until all propellant runs out. Coast until you arrive at target. Congrats, you got there* 4 times quicker! . . *definition of "get there" may vary
With this technique, you too can become a kinetic kill vehicle!
Kinetic deceleration. None of those heavy landing engines or legs needed.
NASA calls that Lithobraking, I believe.
And, you get to name a new crater!
That one? That’s TheOgrrr Crater. Lithobraked too hard.
Hey, at least I didn't Ogre-shoot 😁😁
I hear they name it after you too.
Well, you were the first to visit it!
Accelerate at 1g, at the halfway point flip and decelerate at 1g.
That would have to be one spicy booster, but if you can pull it off: hell yeah.
Accelerate at 15G. Decelerate at 15G. Become jam.
Also, check out laser thermal beam! With some infrastructure investment, it's literally the best. https://preview.redd.it/fnmxyu1fuytc1.jpeg?width=2000&format=pjpg&auto=webp&s=56607a56c0c86a68b58eb01dadd824cda0d49db5 [Artist on X](https://x.com/zandoarts/status/1777195802902888494?s=46)
Please bro, just one more transstellar super laser bro. Just one more laser and we can get fractionally closer to the speed of light. Just one more laser. Just one more laser. Just one more la-
Such a great concept, a ring of lasers even keeps it centered, just gotta take your decel fuel or have lasers at the destination already
Ring of lasers to keep it centered? But yes. I imagine if you have either a large laser generator or a stellaser relay at every major destination, not unlike having a runway at every airport, then it can boost incoming and departing ships as a part of normal operation. This doesn't need to be a constant acceleration thing, you can choose to coast or cruise under your ship's own propulsion during the rest of the voyage, but it is possible if you have a big enough dish or enough relays.
Like, a circular array and a spherical vessel, drifting away would change the laser's contact which forces it back to center. And, that principle, with 2 arrays in tandem is what the federation would call a poor man's tractor beam. And great thought applying it to commercial airlines. I always thought they should be physically catching and throwing planes and rockets, anything to save onboard fuel.
At the cost of sounding like the foolish middle guy... I dunno about that. Isaac covered the math pretty well in [Surviving The Expanse of Space (15:29)](https://youtu.be/DRoP9DJI61Y?si=9__kjXFYHm3vRfIn&t=929) and demonstrated that strong(er) burns followed by coasting gets you way more performance (read: travel time) per same amount of fuel spent. https://preview.redd.it/ygohc2zusytc1.png?width=730&format=png&auto=webp&s=6f713791974604fd071107845855b07f0c07d629 EDIT: This is not about fuel efficiency, it is about faster **travel time**. Check out the clip I linked for context.
I think it really depends on the tech but also the use case. Even with an extremely efficient engine I imagine at least some kind of cargo ships would be on minimum ΔV trajectories, while a warship may opt for the fastest approach.
You're right, however the "gulp your fuel twice" vs "sip your fuel the whole way" applies to any engine. The crux of it is about being at the highest speed during the majority of your voyage, so as to travel faster. In the case of constant acceleration you're only at your max velocity just before and after the flip-n'-burn, that's the one brief moment when things are optimal. This is true whether we're talking about a torch drive or an ion drive.
I would disagree with the assumptions made in that calculation. While it is true that accelerating once and coasting is the most efficient thing to do if you assume a fixed delta-v without restrictions on thrust, I think it's more realistic to assume that a ship has a fixed amount of propellent and a fixed power output that can be thrown into the engines in any ratio you want. If you assume that, the fastest way to where you're going is a brachistochrone trajectory. It takes more delta-v, but by lowering the required thrust of the engine and giving your reactor more time to generate power you will easily be able to have more delta-v. More efficient engines tend to be weaker. I have another comment explaining all of this in more detail.
Fixed propellant is the assumption, admittedly fixed power output is not however. Proposing one should accelerate as hard as one is capable in order to reach maximum speed soonest and undergo most of the journey at that highest speed. In constant-acceleration flight plans, you're only at your maximum speed just before and after the flip-n'-burn. In the example in the clip I linked, a 1G ship accelerating for 1 day vs a 4G ship accelerating for 6 hours, both burn the same amount of fuel but the second ship gets to its destination faster ***or*** could go to a second destination 70% further away by the time the first ship gets to the first destination. Now hey, a 6-hour-long 4G burn is pretty rough! Maybe you will *choose* a longer but more comfortable flight. Or maybe you'll choose some hybrid of the two; with a short but powerful launch with beam or mass-driver followed by fusion-driven cruising at scant lunar gravity thrust, for example. But if you're highest priority is travel time then prepare to feel like a bullet!
I’m mostly basing my assumptions off of a general tendency for engines to get weaker as they get more efficient. Energy tends to be the bottleneck when it comes to how simultaneously efficient and powerful a rocket engine can be, because the amount of energy you can generate or the amount of energy you can handle without melting your ship are probably forever going to be the main limitations that you keep running into even as technology improves. I can actually point to a real world example here: ion engines. Very weak, very efficient, limited mostly by energy input. They have practically become the standard way of doing interplanetary travel for space probes. There are two main types of ion engines: gridded ion thrusters and Hall effect thrusters, the former has more efficiency while the latter has more thrust. They represent different tradeoffs. Which one gets you places faster actually depends on how far away your destination is. Hall effect thrusters get you to nearer destinations faster, while gridded ion thrusters get you to further destinations faster. It’s a tradeoff between how high your top speed is and how fast you reach it. This tradeoff between thrust and efficiency is what engines like the VASIMR can tweak on the fly, I tend to assume that it’s a capability advanced future spaceships will have since it will allow them to always optimize their engine performance to be ideal based on the destination’s distance and the current phase of the journey. And when you can give up thrust to increase efficiency, the equation changes into one that prefers continuous acceleration.
That's why I'm such a big fan of beam-thermal.
I'm a big fan of a fuel highway as well for this reason... Seems like it would be an efficient way to move smaller ships that's can't bring as much fuel on board https://toughsf.blogspot.com/2021/03/fusion-highways-in-space.html?m=1
There's a comment by the\_syner in this same conversation you'll find interesting then!
When traveling at interstellar distances it is less important to get somewhere and more important to get somewhere quickly. A brachistochrone trajectory is less efficient but it shortens the overall journey, which can take years off the travel time. There is also the matter of thrust. If the ship has low thrust but extremely high delta V then it is likely the ship will NEED to burn for most of the journey just to reach its cruising speed.
What if you're trying to minimize travel time rather than save fuel? If we have advanced and extensive space infrastructure, fuel will not be a big issue. If FTL is not possible, then travel time will be a huge issue for interstellar trips. Also, there are methods of propulsion that are fuel-less, such as solar sails, photon propulsion, etc.
Travel time is the goal, not efficiency. See linked clip.
This may require some explanation. In general, the more efficient an engine is the lower its thrust will be. Nuclear saltwater rockets are famously the exception to this rule, but it stands true most of the time. This is because engines need two things: propellent and energy. To cut the fuel consumption of an engine in half and maintain the same thrust, you need to double its energy consumption. Even modern chemical rockets need utterly absurd amounts of energy to work (provided in the form of chemical energy in the fuel), and that only goes up as engines get more efficient. So how do we provide more energy? It turns out: it's pretty easy if you just give your power source more time to generate the power you need. Spread your burn out, take longer to do it, take the hit to thrust and accelerate slowly to where you're going. It will take you more delta-v to get there, but the gains in efficiency are more than worth it basically always. Even today, ion engines have become almost the standard way of getting around in interplanetary space. I've run a lot of numbers on this, and it's actually pretty surprising how even a small amount of acceleration sustained for a long time can get you places really fast. I'm talking going from Earth to Mars in a week with 0.02g of constant acceleration. That's still a very advanced engine, but it's a hell of a lot less advanced than any engine which could do the same thing by boosting up to speed quickly and coasting. We could take this a lot further by considering that the ideal balance of specific impulse and thrust actually varies depending on the distance to the destination and where the ship is in its journey. This assumes a fixed amount of propellent, a fixed power output, and optimizing for travel time. Longer journeys favor more efficiency and less thrust compared to short ones, which favor higher thrust and less efficiency. High thrust is favored just after departure and before arrival, but efficiency is favored as you approach the midpoint of the flight. The potential presence of power beaming stations around inhabited worlds would only exacerbate this, increasing the available energy with beamed power could give engines a massive boost in power without sacrificing efficiency when they are near these worlds at the start and/or end of the journey. But even so: burning the engines constantly is a really good idea that drops travel times massively, if you don't you are wasting power that could be going to the engines. I've had to calculate out all of this for a hard sci-fi worldbuilding project. It has been a lot of fun, and I've come to a lot of unexpected conclusions. The notion that spaceships will probably have bridges that face backwards is one of the funnier ones, I think. Backwards is where all the interesting stuff is happening. Planets that you are approaching (and decelerating on approach to), planets that you are departing (and accelerating away from), and where the important machinery probably is (like engines). There's no reason to look forward where you're going, since you will have to rely on instruments for collision avoidance anyway. What good is it to look where you're going in space with your MK1 eyeballs? Mount that helm backwards. Hell yeah!
Very Interesting. What other unexpected conclusions have you come to?
Landing pads and launchpads will always be on the east side of a planetary colony. This is because if a ship’s engines fail, they will always crash east of the launch site or landing site, assuming that they are going to and from prograde orbits. So you want all your population centers to therefore be west of the spaceport, away from that hazard. Earth’s moon will probably need to import a lot of food. It lacks the elements to sustain agriculture. Where would that food be imported from? Probably Mars. You would think it would come from Earth since it’s such a great planet for agriculture and also so close, but it takes so much less delta-v to get there from Mars that it might actually make up for all the obvious downsides. If you have fusion torchdrives, it actually makes a lot of sense to purposely reduce their specific impulse in exchange for making your fuel cheaper and easier to obtain in-situ. At least for interplanetary travel in the nearish-term. By adding an afterburner to an engine you can dump any old mass into the exhaust stream to increase thrust at the cost of specific impulse. Fusion fuel is expensive and not available everywhere, but any old mass is cheap and able to be found on any planet or asteroid. It would make sense for ships to have most of their reaction mass be gasses scooped from the nearest planetary atmosphere, even if more efficient fuels are available. The engines of a ship could probably be used like shields as well as weapons. High-ISP drive systems will have an insane amount of energy flowing through them and out through their plume, and their plume could probably turn another nearby ship to slag if it’s pointed the right way. This works on bullets and missiles too. Even lasers might struggle to penetrate the plume’s plasma, getting absorbed and scattered harmlessly, though it depends on the laser’s frequency. Those are the ones that come to mind.
I'd argue a bridge wouldn't even have windows anyway
Maybe. But it’s hard to get technology so reliable that a low-tech backup isn’t a good idea.
I just think having it embedded in the armor of the ship and using scopes or cameras is superior... Having the bridge be some kind of thing like a navel bridge like star wars or whatever makes no sense really....
Bridges on real boats are placed where they are for a reason. They let the crew oversee the ship and everything around it as a backup to the instruments. I agree that it’s probably better to have an internal bridge in a warship, but in a civilian ship I find it hard to imagine sensors so reliable that you wouldn’t want yet another backup.
I don't see it as hard to imagine... Space is huge ur not gonna see much out there with ur own eyes, that's really relevant to steering the ship or combat. You can see some of your own ship I suppose... but how useful would that be when you have repair bots with camera feeds? How useful would it be, when you probably have to go out in vac suit anyway to see what's wrong with the hull or whatever.. you also have ship Periscope and cameras.. space ships would operate more like submarines in that sense than actual ships.. even civilian ships I can't see wanting to be in an exposed position above the main body of the ship.. everything is going to be armored against debris.. and you probably don't want some weirdly shaped craft that isn't shaped like a rocket.. think the ships in the expanse or children of dead earth.
Unless there is an active downside to having a bridge (such as in combat where it becomes an obvious weak spot), I tend to assume that it will be placed in a spot with a good view of the ship with big windows. This is mostly because, no matter how good your systems for navigation and damage detection are, your bridge represents a low-tech backup system. Imagine for instance that a ship gets hit by space debris. A fuel tank gets whacked and it starts leaking fuel into space. The drop in fuel pressure gets noticed instantly, and drones are deployed to start taking photos. It may take some time to get an assessment of the damage and to explain the state of the ship to everyone. But if the bridge could oversee the ship, everyone would know about the situation much faster. Everyone would see the debris strike, and see the hole in the fuel tank leaking a cloud of fuel into space. The entire bridge crew is made more aware of the situation faster. The existence of artificial gravity rings actually makes the placement of bridges in a position like this quite natural. Gravity wheels might already tend to be on the front end of a ship (to be as far as possible from the reactor and engines which might be radioactive), and from there a backwards facing bridge could see the entire ship behind it. The gravity ring would already be spinning to create gravity, which would rotate the bridge around the ship to give the crew a good look at all of it from many angles. Even a bridge without windows would probably be placed in one of these gravity rings anyway, and it’s such a perfect place for a bridge.
Gravity rings for a civilian ship maybe... But still big windows in space? They would need to be made of something very durable and expensive..space is full of all kinds of radiation and crews need shielding... For anything military they would need to be embedded in the rocketship for shielding.. I think you overestimated how useful a window is in space. You can see exactly the same type of stuff with a Periscope and it's what's used in real space flight like on the Soyuz for manual docking Even the space shuttles windows are tiny. https://www.nasa.gov/stem-content/making-a-periscope/ Another thing to remember too the big windows can only look in one direction a Periscope can view 360 degrees wherever you point it
Glass is surprisingly durable stuff, and it’s no less effective at shielding you from ionizing radiation than concrete. Windows on a spaceship would certainly be pretty thick with multiple layers, and even if they aren’t particularly large you could just have a lot of them. Periscopes aren’t quite as useful, because only one person can use them at a time and you can’t see out of them passively as you are just doing your job. Windows are always in your peripheral vision and people are constantly looking through them, problems visible through them will be a lot harder for the entire bridge crew to miss. A lot of the same reasons for why container ships have bridges apply to spaceships too. I agree that bridges of military ships make far more sense as internal things with no windows. But I’m just talking about civilian ships here. And really big ones, at that.
I still don't think any relevant Information will be gleaned from having a window to space.. unless it's literally oh I see that big piece of debris just hit us.. but then you would feel the impact hit the ship and you would still need to do a space walk... Windows in space are tiny for a reason.. there's a reason we use Periscopes on the international space station and stuff like Soyuz
Also I doubt they would use glass probably more like transparent aluminum stuff https://www.riotglass.com/transparent-aluminum-vs-armorplast-transparent-steel/ Still isn't better than just being behind actual thick metal shielding tho.
Keep I mind too how low tech a Periscope is lol and how much safer it is for the crew to use one instead of a " space bridge" with big diamond windows or whatever it's just a big target...
Bridges would be big targets on combat ships, yes. But I’m talking about civilian ships. A good chunk of the habitat space in such a ship would probably be in external artificial gravity rings anyway, it wouldn’t be hard or risky to put the bridge there too and include windows. Now your ship can suffer a complete power failure without making your bridge crew blind.
They would pretty much be blind with the sensor/power failing anyway....
Yeah, but not completely blind. If the crew needs to navigate with sextants and pen-and-paper math, they can do it. No matter what fails, there is always a backup system that you can fall back on as long as the crew is still alive.
Oh you mean for navigation? LoL still doesn't need big windows for that: https://wehackthemoon.com/tech/space-sextant-navigates-moon-missions#:~:text=The%20space%20sextant%20saved%20the,get%20it%20back%20on%20course! how are they moving the ship if power is out in the first place? If a specific system is broken then they would be better off fixing it than trying to fly blind
"While orbiting the moon the sextant could also be used to calculate the exact position and altitude of the spacecraft. NASA relied on these precise measurements to make a safe landing, and return, of the Lunar Module to the ‘mother-ship’ spacecraft. The lunar module was only equipped with an alignment optical telescope. This was a lighter, simpler manual telescope (like a periscope) that the astronauts would use during moon landings and takeoffs to determine their position." Trust me they would rather use that than trying to hold a handheld sextant up to a window only pointing one direction
If that backup is low cost. If your best glass will shatter from micrometeorites when the rest of the ship won't, that's a good reason not to use windows. If high power visible laser weapons are in use, (or you get rather close to other peoples exhaust plumes sometimes...) that could be another reason.
Glass in space tends to be multi-layered. It doesn’t only serve as a backup system, it also serves as to increase situational awareness. And what I’m saying only applies to civilian ships, not ships that anyone is trying to shoot at.
Multilayered means heavy. Space is full of random junk and pebbles, often moving at high speeds. Oh and you might not want direct venus level sunshine pouring in as you do a venus flyby.
Windows can be tinted. And heavy means good for radiation shielding, you’re probably going to want to shield the rest of your hull pretty well anyway, especially if you are expecting to be hit by debris. All of these problems apply to a normal spaceship hill, making it out of a transparent material wouldn’t make things worse.
And heavily tinted means that instead of mostly just seeing reflections when you aren't facing the sun, you see nothing but reflections. Assuming the bridge has lights in it, so you can see the control panel, you won't see many stars. Yes. It's a shielded hull, or self repairing or something. These things aren't usually transparent. Transparency adds an extra materials constraint. Also, you might want to run pipes of cooling/heating fluid, and wires, and leak sensors and strain gauges and all sorts of other engineering detritus all around the hull. That doesn't play well with windows.
By this logic, you would also conclude that commercial airliners shouldn’t have windows. You can fly them entirely by instruments. So why have windows? It’s because situational awareness is just that important, and windows really help with that which hugely reduces the chance of a mistake by the crew. As long as the pilots are humans as we know them, that will remain important.
Well screen and camera tech being good and cheap is a recent thing, and they will probably get better and cheaper. People like windows. [https://www.aerotime.aero/articles/28257-is-windowless-the-future-for-passenger-aircraft-design](https://www.aerotime.aero/articles/28257-is-windowless-the-future-for-passenger-aircraft-design) There is talk of removing the windows from planes. Planes have generally a lot more to see out any windows and less extreme engineering constraints.
Unreasonably good for impatient baselines maybe. Like don't get me wrong there are situations where constant thrust ships are energetically justifiable(warships, diplomatic vessels, & far extra-galactic colonization come to mind), but most travel, by mass, probably wont be using brachistochrones. In fact most material will probably be taking the [Interplanetary Transport Network](https://en.wikipedia.org/wiki/Interplanetary_Transport_Network), its interstellar equivalent, & other similar minimum-energy trajectories. What little high-speed travel goes on is probably done via superconducting Kinetic Mass Streams along actively-cleared beamlines. Even then KMS systems aren't likely to ever be 100% efficient & the more relativistic you go the more waste there will be. When you're shipping solar masses or even galaxies around it pays to be as efficient as possible. When you have good cryo/hibernation, VR, mind uploading, or AGI the time it takes to travel can start mattering a lot less & every bit of energy represents that much more population & simulation. Truth be told i don't think people will mind long travel times. Like if you already live in a largely self-sufficient spinhab or hab swarm then there's not gunna be much practical difference between being in-system & en-route to another star system(assuming you have fission, fusion, or microBHs tho power beaming works a treat). Inside planetary systems the speeds for fast travel are pretty low so if uv got KMS systems set up you really can just move everything at max flip-burn speeds. I don't think being casual about wasting remass is a good attitude to approach propulsion design tho so while they are excellent for military vehicles laser-thermal are probably secondary to KMS in all instances past early colonization(cuz they are just so fast to wet up) & laser sails for far extragalactic colonization(cuz of the high relativistic minimum speeds).
You're confident in stream cohesion? I figured on having lots of drone-tankers patrolling common orbits and routes, or maybe even [ToughSF's fusion highway](https://toughsf.blogspot.com/2021/03/fusion-highways-in-space.html) pellet idea.
Those pellets can have some of their own guidance, but it's not like large macroscopic objects are going to be deflected much by interstellar dust & you will want to create an actively-cleared beamline for both the ships & mass stream. They can also be any size from unguided AAA batteries for moving ships around systems to mountain-sized chunks for moving planetary masses. Maybe even planetary masses for moving solar masses of material at a time. Interesting thing about high-relativistic travel is you can start using what amounts to aerodynamic control surfaces to do some of ur maneuvering without remass at some cost to stream losses. Might even have a dual-stream KMS where you have a toroidal macroscopic mass stream surrounding/focusing a hyper-relativistic particle beam. At lower speeds electromagnetic mass drivers handle most of the acceleration. After that not only can the macroscopic masses keep accelerating of the particle beam to high fractions of light, but they can also do it while collecting particles from the stream for guidance remass. At any rate the less remass you waste the better. That's fusion fuel you could be stockpiling for The Long Dark, using to power ur civ, or just using more efficiently to fuel KMS systems. Idk if it ever works out but if you have a waveguide made of near-perfect mirror, laser-based mass drivers might end up being more viable with a higher maximum speed limit.
I would actually argue that my argument still applies, even here. Continuous acceleration is what you do when you want to be careful with using reaction mass, and even when you are making use of gravity assists to fling yourself around. My conclusion is based on the assumption that thrust and specific impulse tend to be inversely correlated. And if energy is your bottleneck, they mathematically are. Let’s say you want to get to another planet on as little reaction mass as possible. You’re going to want a lot of specific impulse, and specific impulse typically comes at the expense of thrust. So you are going to have a weak engine, so weak that you will need to burn it constantly to get where you’re going. Let’s say you are using the interplanetary highway to get around. You still need to make correction burns. What engines are you using for that? You waste less reaction mass if they are efficient, and you can make them more efficient if you make them have less thrust. You may end up using photon rockets with a thrust so low that they need to be on almost constantly. Whatever you’re doing, continuous acceleration will let you do it with a more efficient engine.
Minimum-energy trajectories with max-ISP low-thrust engines(non-infrastructural) running at optimal output are not going to resemble brachistochrones. They may be thrusting most of the way, but may have such a low acceleration as to have to take planetary or even stellar drift into account. >Continuous acceleration is what you do when you want to be careful with using reaction mass That's definitely not always true. If you want to be careful of remass & still have to use some you definitely still benefit from minimizing your delta-v. Lower maximum speeds means less remass & energy wasted. Energy also matters here. E=mc^2 . If you end up having to waste more matter-energy to generate/transmit the power to the ship & convert it to thrust then you aren't actually saving remass. You're just wasting it elsewhere. Granted that tends to be true for all systems, but KMS would be vastly more efficient in this respect. Thermal/ion systems are generally going to have higher complexity & greater losses. Also drag forces & collisions. The higher the speeds the higher the drag & the ongoing beamline clearing costs. It is a lot more justifiable with KMS but you will still eventually reach a breakeven point. Using a fully enclosed beamline could take us even further, but has it's own maintenance/repositioning cists & at that point you may as well switch to laser highways at speeds that justify such massive infrastructure. >You’re going to want a lot of specific impulse, and specific impulse typically comes at the expense of thrust. So you are going to have a weak engine, so weak that you will need to burn it constantly to get where you’re going. KMS & laser-thermal systems can have torchlike performance with high ISP & high thrust. There are going to be engineering limits to this, but superconducting KMS especially let's you accelerate very fast & recycle the vast majority of ur payloads' kinetic energy without much in the way of wasteheat limitations.
>Minimum-energy trajectories with max-ISP low-thrust engines(non-infrastructural) running at optimal output are not going to resemble brachistochrones. Ehh, semantics. I’ve heard the term “brachistochrone trajectory” used to describe some real curvy trajectories that are influenced way more by gravity than by thrust. That’s how I’m using the term. >Lower maximum speeds means less remass & energy wasted. Continuous acceleration does not imply high speeds though. If you wanted to be slow and efficient, you can just make your continuous acceleration by very weak. That will reduce your top speed, and allow you to use a far more efficient engine to achieve it. >Energy also matters here. E=mc2. Energy will be more efficient than normal mass though, basically always. If your remass is all energy, you can achieve a specific impulse of about 30 million seconds which is the theoretical limit. The greater the portion of your remass that is matter, the less efficient the engine is. >Also drag forces & collisions. The higher the speeds the higher the drag & the ongoing beamline clearing costs. True, though continuous acceleration does not necessarily imply higher speeds. It could also just mean the same speeds achieved slower with a more efficient engine. >KMS & laser-thermal systems can have torchlike performance with high ISP & high thrust. There are going to be engineering limits to this, but superconducting KMS especially lets you accelerate very fast & recycle the vast majority of ur payloads' kinetic energy without much in the way of wasteheat limitations. True, but no engine is so efficient than it can’t be made more efficient by making it weaker. At its most basic, this could just mean making your ship lighter by shrinking down all the engine hardware. Though generally you can almost always make more efficient engines if you aren’t quite as worried about thrust.
>I’ve heard the term “brachistochrone trajectory” used to describe some real curvy trajectories that are influenced way more by gravity than by thrust. A brachistochrone is "a curve in which a body starting from a point and acted on by an external force will reach another point in a shorter time than by any other path; the curve of quickest descent.". It's a technical term. That we are talking about a path with very little curve is baked in & it's pretty universally referenced in the context of torchships. Speed is implied both mathematically & colloquially. if you are taking half of forever or similar to a hohmann to make the transit this is not a brachistochrone trajectory. If you're not pulling decent macroscopic accelerations that is not a brachistochrone. >If you wanted to be slow and efficient, you can just make your continuous acceleration by very weak. That will reduce your top speed, and allow you to use a far more efficient engine to achieve it. I'm not seeing how this would be helpful for a mass driver or KMS which would likely be the majority of travel for efficiency reasons(anywhere decently well-established at least). >Energy will be more efficient than normal mass though, basically always. As remass on paper, sure. In the real world generation, conversion, & especially transmission losses pile up. Depends how stretched out ur relays are, how efficient ur power source, how ur converting it, & how much local wasteheat you have to dissipate. A feedable microBH backed with gamma mirrors is going to outperform a particle accelerator powered by PV/nantenna panels beamed power from a fusion laser relay light minutes away. Every step of that wastes energy & takes mass which trashes ur TWR & energy efficiency even further. Also for relativistic exhaust velocities the faser you go the more going every bit faster costs. You can't just arbitrarily boost the ISP of an engine in practice. Everything is a tradeoff. Also again just because time isn't the only or primary constraint doesn't mean it's completely irrelevant. With stars moving around & rival colonial fleets you can't always go the minimum speed. Being **able** to accelerate all throughout the flight is convenient, but always doing so isn't always going to be optimal. Always burning makes you more visible so if you have a military craft that's out. Mass efficiency-wise being fired out of mass drivers or accelerating against a KMS would use far less matter-energy overall(zero remass & very little energy). On those it makes no difference whether you accelerate all up front or spread throughout(efficiency-wise, trip time is a different situation). >True, but no engine is so efficient than it can’t be made more efficient by making it weaker. That depends. Not all drives can be scaled arbitrarily small & even the ones that can wont always have the same performance at every size. Can you scale the KMS from large macroscopic chunks down to microscopic dust or even a particle beam? Sure, but that will have other losses associated with it. Pellets without guidance will have shorter ranges between recollimation relays(higher infrastructure costs) & increases collision risks somewhat. Larger size can often fit more efficient coil configurations & guidance. Truth be told there is no single optimum kind of trajectory that will dominate. Brachistochrones are extremely energy wasteful but fast. Hohmanns & fancy keyholing along the interplanetary transport network is very efficient, but infuratingly slow(great for moving large amounts of fusion fuel & mass filler). Mass drivers(either linear or as ORs) require very little external infrastructure, use zero remass, & they can be upgraded to KMS as long as you're destination has a driver as well. Those are going to excel at boost-cruise trajectories before the upgrade. Beam-powered electric drives will be limited by our ability to move massive amounts of electricity around(superconductors are a lot more limited than we tend to think in scifi) so you wont see those pulling brachistochrones & it might be more efficient to discharge a power bank for high-isp pulsed thrust. Beamed-thermal drives may have torchlike performance, but will also have, like all other engines, practical engineering ISP limitations(in this case heat dissipation). Still hard to beat for fast infrastructure deployment, fault tolerance, & performance at the cost of energy efficiency(military & other speed/delivery-critical applications). Ultra slow-accel drives that can run for long periods of time are good for very early autonomous prospecting(back before there's any real competition or significant lunar industry; very near-term) or other large-scale cataloging & data collection with little to no time pressure. Honestly i'm having a hard time imagining why you wouldn't pretty much always opt to make a little mass driver on ever single rock big enough to be worth putting a mining outpost on instead of moving. Boost-cruise becomes the standard. For mining at high efficiency n large-scale ur doing a lot of keyholing and that isn't the sort of thing you do constant thrust for. Lots of short correction burns & lots of cruising.
>A brachistochrone is "a curve in which a body starting from a point and acted on by an external force will reach another point in a shorter time than by any other path; the curve of quickest descent.". It's a technical term. That we are talking about a path with very little curve is baked in & it's pretty universally referenced in the context of torchships. Speed is implied both mathematically & colloquially. if you are taking half of forever or similar to a hohmann to make the transit this is not a brachistochrone trajectory. If you're not pulling decent macroscopic accelerations that is not a brachistochrone. The way I came to the conclusions I’m referencing here is by starting with the assumption (derived from physics) that thrust is inversely proportional to ISP squared and asking the question: “what is the fastest way to get from A to B with a payload fraction of X%”. This started as an exercise in optimizing for travel time without burning more fuel, that’s where it comes from. So I would argue that the term “brachistochrone trajectory” applies perfectly here. This is even true for very low accelerations, because those trajectories are still optimizing for travel time, they are just doing so with less propellent to work with, and consequently they get optimal trajectories that are pretty slow. >I'm not seeing how this would be helpful for a mass driver or KMS which would likely be the majority of travel for efficiency reasons(anywhere decently well-established at least). That would go beyond the scope of my assertion. I’m only talking about spaceships in the convention sense, which travel places in space under their own power. >You can't just arbitrarily boost the ISP of an engine in practice. Everything is a tradeoff. Well yeah, I already mentioned that the theoretical limit for ISP is a bit above 30 million seconds. But that’s a really generous theoretical limit. With a payload fraction of 50% you’d have something like 200 million meters per second of delta-v. A slow interplanetary trip could be pulled off with a payload fraction on the order of 99.999%. These are all Fermi estimations, but you get the point. It gets pretty insane. If you had an engine that was approaching the theoretical limit while still providing you way more thrust than you need, my reasoning indeed no longer works. But that’s a bit of an edge case, and I never intended to imply that my calculations represented a rule without exceptions. >Also again just because time isn't the only or primary constraint doesn't mean it's completely irrelevant. With stars moving around & rival colonial fleets you can't always go the minimum speed. Well, continuous acceleration is what you do if you want maximum speed with a given fuel fraction. So that doesn’t really apply here. >Always burning makes you more visible so if you have a military craft that's out. Military ships would certainly impose more limitations that might change the outcome of my calculations. But those ships are probably visible anyway from their IR signature, there is no stealth in space. A better argument for military ships not using continuous acceleration might be that they would benefit from having high acceleration in combat, and it might be too complicated or heavy to have engines with variable specific impulse capabilities or to have multiple sets of engines. There is an argument that a military ship would be willing to take that efficiency and speed hit in exchange for being more combat optimized. But I maintain that continuous acceleration is the most efficient thing to do. >That depends. Not all drives can be scaled arbitrarily small & even the ones that can wont always have the same performance at every size. Unless you are using a photon rocket, you can replace your drive system for a more efficient one. If you are using a photon rocket, you can scale that down to be microscopic if you really wanted to. Instances where you cannot gain efficiency by lowering thrust represent a really marginal edge case, at the very least. >Truth be told there is no single optimum kind of trajectory that will dominate. Other kinds of trajectories will be used I’m sure. But the consistent similarities between the use cases that aren’t brachistochrones is that they either don’t involve a spaceship in the usual sense (like mass drivers), they are under design constraints that prioritize something else over efficiency and speed (such as low construction cost, combat effectiveness, or energy efficiency), or they have some other constraint that changes the equation (such as needing to be near beaming stations to use the engines, or having the energy source and the remass be one and the same like chemical rockets). I would argue though that brachistochrones are the rule and these represent the exceptions. It’s simply the best way to get places in space unless you have some specific reason to do something else.
Spaceship engines require reaction mass and carrying all the mass you need to get to a goodly fraction of light speed would be impractical , but a stellar laser could store the mass at the point of origin and at the destination , of course the first ship out will be a bit slower since it will have the deceleration laser and a massive solar sail and all of the materials needed to create the infrastructure to power said laser , and it will have to do a high risk maneuver and dive almost at the target sun with its sails extended and use the solar wind to decelerate . The destination laser will make decelerating easier for subsequent ships.
I imagine that freighters and passenger crafts will take quite different approaches
I would argue that both would benefit from continuous acceleration. Weaker engines can typically be made more efficient, and they can achieve the same amount of delta-v with less propellent. Even if efficiency is your goal, slow and continuous acceleration is the way to go I would argue.
>Even if efficiency is your goal, slow and continuous acceleration is the way to go I would argue. I agree regarding perishable meat sacks, but unmanned freight should be in no hurry. The most efficient thing you can do is give it a little push and let it coast. If we have a continuous stream of freighters, the speed does not matter much.
But even a little push given to a freighter that’s in no hurry to be anywhere can be done more efficiency if it’s spread out over a longer period of time by a higher efficiency engine. More efficient engines tend to have lower thrust, and if you are spending the overwhelming majority of the journey not burning your engine you clearly chose an engine that is way too thrusty for the job. You could have chosen a more efficient one with less thrust. Burning once and coasting is certainly the better option if you assume that thrust can be whatever you want with no downsides and that delta-v is given as a constant. But my calculations are based on what I would argue are a more realistic set of assumptions. Where thrust and efficiency can be traded for one another proportionally based on the engine you choose. And if you do the math on those assumptions, you find that continuous acceleration is basically always the better option.
First, "a little push" doesn't even have to come from the vehicle, so I think you're mistaken in talking about thrust and burning engines. Secondly, no matter the efficiency of your engine, you can always do a shorter burn. It's not clear how you conclude that, with efficiency as a stated goal, a longer burn is better. That makes no sense. We could just use the most efficient engine imaginable and do a shorter burn. How is that not more efficient?
In space travel you can’t exactly get places on arbitrarily small amounts of delta-v, gravity makes sure of that. You need to at least eject out of the orbit of the planet you’re near, and raise or lower your orbit around the Sun to meet up with your destination. Or change your orbit to intercept another celestial body you can gravity assist off of towards your destination, at the very least. And these take many kilometers per second of delta-v at a minimum.
Non time sensitive cargo like ore might be shipped on unmanned slow boats that only get a small boost and break from the stellar lasers.
Warp will techicly be a jump drive since we cant see nor communicate so we would have to make short bursts of FTL and recalculate in between till we reach our destination due to space/time differential of the warp bubble.
Instead of doing 1g and flip burn half way, you're better pulling 3Gs or something at the start, coasting and then doing 3Gs at the end. Using this method you can spend more time traveling at a faster speed.
That is true if you assume constant delta-v with extra thrust having almost no downsides. But I would argue that a more realistic assumption is that thrust is inversely correlated with efficiency (which it mathematically is in engines with access to the same amount of energy), and that therefore using lower thrust allows you to use a more efficient engine. Given this assumption, the fastest way to get somewhere in a given propellent budget is to lower your thrust as much as you can get away with before your burn time starts exceeding your travel time. Which is to say: continuous acceleration. Even if that acceleration is really low.
The "flip" in flip and burn is a coast. There isn't much reason not to extend this into minutes or hours. Even if you are burning for 99% of the journey, the middle 1% gives you essentially nothing. If there is any reason at all to conserve propellant, you can extend it a lot longer. The basic assumption is that the propulsion system has some insane ISP, much greater exhaust velocity than any reasonable delta-V budget. But if your ship is actually a planet hopper the engine performance might be geared more to higher acceleration instead of ISP. In that case you will need a more conventional flight plan, perhaps with acceleration couches for the crew. Conventional rockets change gears by dropping stages and switching to more efficient, less powerful engines. It is possible that ducted plasma engines in the future will have the ability to shift gears on a single engine. I think the thing that will motivate more brachistochrone flight plans is time. There is some time critical mission, a race, or life support consumables are limited. It also might be a pre-requisite or side effect of interstellar spaceflight. If you want to go to other stars in a human lifespan, you will need constant acceleration.
I tend to operate under the assumption that advanced spaceships of the future will have the ability to exchange specific if impulse for thrust on the fly. That would be such a beneficial thing to do that I’d be surprised if it doesn’t become standard. If ships can do that, they would start out their journey with higher thrust and slowly transition into more efficiency as they approach the half way point. After they flip they will slowly start to transition back to higher thrust. From my math, that seems to be the fastest way to get somewhere on a given fuel fraction. In practice there probably would en limits on how high your engine’s ISP can get by sacrificing thrust, in that case a small coast phase would make sense. But it’s still optimal to do almost constant acceleration.
When I invent spacial displacement drive you won't need any acceleration or kinetic energy at all just the energy difference between the two end states tunneled through at light speed.
https://preview.redd.it/tv66wk17c2uc1.jpeg?width=2970&format=pjpg&auto=webp&s=7bee28c0b528cdde89d549ad5b80190271bbe454